Leslie Pfenninger, the 21-year-old Lanham woman who wants to know why she can't get a government job here in federal city may not like the answers she's likely to get wher her would-be boss, Jimmy Carter, calls back.

The President's aides haven't come up yet with a reason. But it may be that Carter will end up telling the college graduate (with two bachelor's degrees earned in three years) that she was born in the wrong place, or at the wrong time, or is of the wong sex or that she got comparatively low marks on her civil service exam.

Pfenninger is one of the people got through to Carter on Saturday when he held the first-over talk show between a President and the public. Millions called the toll-free number and 42 got a chance to ask the President anything they wanted. She wanted to know why it takes so long to get a job with Uncle Sam? The President said he would find out.

Federal officials frantically are checking to see what, if anything, is the holdup. Odds are it will be for one (or more) of the following items:

Wrong Time: Competition for government jobs is toucher than ever. In some fields, the government now has from 30 to 60 applicants for a single job. And last week the President ordered a partial hiring freeze, making the job outlook even more grim.

Wrong Place: On Feb. 7, this column outlined the problems job seeking here face because one of every seven federal positions is under a state-residency quota that excludes - because their state quotas already are filled - residents of Maryland, the District of Columbia and Virginia. It is possible the position she wants is a "quota" job.

Wrong Sex: Veterans get preference in hiring, on their test scores (points are added), and layoff protection in many cases. Since few women are veterans, they are at a disadvantage in competing with men who are.

Test Score Too Low: She also may be low on the hiring list because her civil service test scores and job rating ae "low" even if she got a relatively high grade. There are so many people seeking government jobs now - there are from 1,000 to 3,000 queries in this area each day - that agencies have their pick of top-scoring candidates and must take them from the top.

None of those answers is official yet. Government aides are still checking for the President to find out what the particular problem or problems are in the Leslie Pfenniger case. But chances are good that at least one, and maybe more, of the points is what is holding up her consideration and/or hiring. Meantime, she's waiting for the telephone to ring.